The basic principle of the function of an FMCW radar sensor (frequency modulated continuous wave) is that the frequency of a transmitted radar signal is modulated in the form of a ramp and the signal reflected by an object and received again by the sensor is mixed with a part of the signal transmitted at the moment of reception to form a baseband signal. The baseband signal then contains a frequency component which corresponds to the difference frequency between the transmitted signal and the received signal. On the one hand, the frequency is a function of the object distance due to the change in transmitting frequency arising during the signal propagation delay, but on the other hand is also a function of the relative velocity of the object due to the Doppler Effect.
The baseband signal is normally segmented by rapid Fourier transformation into its frequency spectrum, and each located object is represented in this spectrum by a peak at a frequency, which is a function of the distance and the relative velocity of the object. In this case, the frequency of a received peak establishes a relationship between the relative velocity and the distance in the form of a linear correlation, corresponding to a straight line in a distance/velocity space. The term “linear” in this case is understood to mean that the correlation identified by it may include a linearity factor and an additive term.
Thus, based on one single frequency obtained, it is not yet possible to clearly determine the actual distance and the actual velocity of an object. For that, it is necessary instead to locate the same object on at least two frequency modulation ramps of the transmitted signal, whereby these two frequency modulation ramps must have different slopes.
The frequencies of the baseband signal obtained in the individual frequency modulation ramps are then associated with objects based on coincidences between the possible values of distance and relative velocity associated with both frequencies. In the case of two straight lines in the distance/velocity space, this corresponds to a point of intersection of the two straight lines. This association of the frequencies or straight lines to potential objects is referred to as matching or frequency matching. With such a comparison of the different linear relationships obtained in the case of the individual frequency modulation ramps, it is possible to calculate relative velocity and distance of a radar object. The FMCW method is particularly efficient when only a few radar objects are detected.
If, however, multiple objects are situated simultaneously in the locating range of the radar sensor, the problem then arises that it may no longer be clearly determined which peak belongs to which object, even when evaluating two modulation ramps. Thus, for example, for a situation involving two objects in the distance/velocity space, also referred to hereinafter as d-v-space, two pairs of parallel straight lines are obtained, which between them form four points of intersection. However, only two of these points of intersection may correspond to real objects, whereas the other points of intersection represent so-called pseudo objects.
Therefore, more than two modulation ramps of differing slope are normally used, for example, four modulation ramps. Real objects may then be detected by the fact that in the d-v-space a coincidence exists between distance/velocity pairs belonging to the frequencies of the baseband signal obtained on the four different frequency ramps.
However, since the frequencies of the peaks may be determined with limited accuracy only, it cannot also be expected for a real object that the four straight lines belonging to the four modulation ramps intersect at exactly one point. Rather, up to six different points of intersection will be obtained, which, however, lie relatively close to one another. When comparing the different linear relationships, as part of the search for a coincidence, a certain tolerance is therefore allowed. The criterion for a real object may, for example, be that all straight lines obtained from the different modulation ramps intersect in one point within the scope of the tolerance limits.
However, in situations involving a plurality of radar targets, such as objects in the form of guardrail posts, or a plurality of motor vehicles in the locating range, for example, motor vehicles at the end of a traffic jam or in parking lots, the effort required to detect the objects is increased. Thus, as the number of objects and number of modulation ramps increases, so too does the computational effort.